Light-emitting diodes (LEDs) are attractive candidates for replacing conventional light sources such as incandescent bulbs and fluorescent tubes. LEDs have significantly greater energy conversion efficiencies than incandescent bulbs and, in some cases, higher energy conversion efficiencies than fluorescent tubes. Furthermore, the conversion efficiencies of LEDs are steadily improving over time, and hence, LEDs will provide significant energy savings in the not too distant future.
In addition, LEDs have lifetimes that are much greater than the lifetime of either fluorescent lights or incandescent bulbs. This advantage is particularly important in applications in which the cost of changing a bulb or fluorescent tube is high. Automobile taillights and traffic signal lights are already being converted to LED-based lighting systems to take advantage of this aspect of LEDs.
Finally, LEDs are “point sources”, and hence, are better suited than fluorescent tubes for lighting applications in which light must be collimated or focused. One such class of applications involves the illumination of a planar light pipe that is used to illuminate a two-dimensional device such as a Liquid crystal display (LCD) or a switch panel. The light pipe is typically a thin sheet of plastic having one or more edges through which light is injected from a light source. In handheld devices such as cellular telephones or PDAs, the thickness of the light pipe is often less than a few millimeters. Hence, the small size of an LED is particularly important in such applications.
The high light conversion efficiencies that make LEDs attractive as replacement candidates depend on providing an environment in which the heat generated by the LED is efficiently removed so that the LED is not subjected to high temperatures. For the purposes of this discussion, the light conversion efficiency of a light source is defined to be the amount of light generated per watt of electricity consumed by the light source. The light conversion efficiency of the LEDs that are currently available decreases rapidly with increasing temperature. In addition to reducing the light conversion efficiency, heat also shortens the lifetime of the LED and can lead to premature total device failure. While the light conversion efficiencies of LEDs are large compared to incandescent light sources, the majority of the power applied to the LED is still converted to heat.
LEDs also age with time. As a result of the aging, the amount of light that is produced for a given current through the LED decreases. In light sources that use LEDs that emit light in different wavelength bands to generate illumination that is perceived as having a particular color, the aging effects lead to a color shift over time in the perceived color. In many applications, the color shift is more objectionable than the decrease in intensity of the light source. The rate at which LEDs age depends on the operating temperature of the LEDs, higher operating temperatures leading to more rapid aging.
Accordingly, packaging arrangements for LED dies must provide an efficient path for removing heat from the dies. Lead frame packages are attractive from a cost point of view. However, lead frame packages that provide sufficient heat dissipation are not available for high power dies. These packages typically rely on moving heat from the LED to an outside heat-dissipating surface since the surface area of the LED package is too small to dissipate heat to the air surrounding the LED. Typically, the heat is transferred to the core of a printed circuit board on which the LED is mounted. In a typical LED lead frame package, the LED is mounted on the internal portion of one of the leads and the heat is moved over that lead to the core of the printed circuit board. Unfortunately, the lead heat path tends to have too high of a thermal resistance, and hence, the die must run at a substantially elevated temperature to force the heat through the lead.